1. Field of the Invention
The present invention relates to a technique for manufacturing metal cans having can body walls with contoured configurations.
2. Related Art
Beverage cans are generally manufactured into two different forms i.e., a three-piece can and a two-piece can. Of these, the three-piece can is manufactured from sheet metal, such as a steel sheet treated electrically with chromic acid or subjected to other surface treatments, by rolling a rectangular piece of such sheet metal (or a blank), soldering, welding or bonding both sides of the blank to form a cylindrical can body, necking an upper portions in the vicinity of both open ends of the cylindrical can body into a reduced diameter, flanging edges at both open ends radially outwardly, double seaming a can end to one of the open ends and attaching another can end to the other open end by double seaming after filled with a beverage product. On the other hand, the two-piece can is typically manufactured from a tinplate sheet or an aluminum alloy sheet by blanking and deep drawing or blanking, drawing and ironing such sheet to form a cylindrical can body with an integral can bottom and open top end, necking an upper portion in the vicinity of the open top end into a reduced diameter, flanging an edge at the open top radially outwardly and attaching a can end to the open end of the can body by double seaming after filled with a beverage product.
In the industry, all these beverage cans are produced massively and relatively economically to substantially an identical shape. As the cans are produced substantially to an identical shape, they can not adequately be discriminated or differentiated from one another by their appearance. As the beverage cans are manufactured massively and relatively economically as above and no comparable beverage packaging in another form is readily available, there are strong desires among beverage manufacturers for economical beverage cans with unique configurations to help differentiate their products.
In their efforts for satisfying such desires of the beverage manufacturers, many can manufacturers have been trying to add improvements to their manufacturing technology and a number of processes for reshaping the can bodies have been proposed to date.
One example of such reshaping processes have been disclosed in Japanese Patent Laid-Open Application No. 28492/1974. This Japanese Patent Laid-Open Application discloses a reshaping process wherein a can body is expanded radially outwardly from its interior and provided with a contour of rigid patterns by a rubber cylinder having a plurality of recesses being disposed circumferentially and a plurality of split segments. As a three piece can body is not subjected to an intensive metal work such as the drawing and ironing operations, it retains sufficient ductility to permit adequate expansion to form a desired contour by the aforementioned reshaping process.
Although a three-piece can manufactured by a conventional method retains sufficient ductility to permit such adequate expansion, a drawn and ironed can having undergone intensive metal works, i.e., drawing and ironing operations, no longer retains adequate ductility so that a conspicuous contour to give desired effects are not attainable thereon.
More specifically, the drawn and ironed can has been ironed substantially to the limit of its formability capability so as to form a very thin can body wall, so that the can body essentially has extremely reduced ductility due to work hardening and residual stress. With the aforementioned reshaping process, therefore, a drawn and ironed can body may be ruptured before a desired contour is formed thereon. While the greater the expansion (or difference in diameter between expanded portions of the can body and an initial can body diameter before expanding operations), the more conspicuous the contour becomes, an expansion of the can body beyond the limit of its elongation may result in tear or rupture of the can body. In order to prevent rupture of the can body, the can body may be reshaped to a lesser extent within the limit of the elongation or the reshaping can take place after the can body is annealed to release the residual stress and restore its ductility, but following disadvantages still remain to be overcome.
Firstly, the reshaping to a lesser extent makes a contour on the can body less conspicuous and adequate effects for product differentiation are not attainable. Secondly, annealing of a drawn and ironed can body will result in reduced physical strength and internal pressure resistance to such extent that the can body may be crashed during necking or double seaming operation, or a bottom dome of the can body may be buckled by the internal pressure created when the can has been filled with a beverage product and sealed. If walls of the can body are made thicker to compensate for the reduced physical strength, there arises another disadvantage that increased mass of metal makes the can body less economical.
Although a three-piece can manufactured by a conventional method retains sufficient ductility to permit adequate expansion by the aforementioned reshaping process, such process causes a disadvantage also with the three-piece can in that the metal in the portions of the three-piece can body surrounded by the expanded portions are pulled by the expanding operation and expanded radially outwardly to an extent that the edges of the expanded portions can not clearly be configured to attain desired conspicuousness.
With the reshaping process disclosed in Japanese Patent Laid-Open Application No. 28492/1974, a can body is expanded radially outwardly from its interior and provided with rigid relief patterns by a rubber cylinder having a plurality of recesses being disposed circumferentially and a plurality of split segments. This process has a disadvantage in that the portions of the three-piece can body surrounded by adjacent relief patterns are forced to expand radially outwardly and adequately conspicuous relief patterns can not be attained on the can body. The closer the adjacent relief patterns are disposed one another, the less conspicuous boundaries of the patterns and the less rigid the patterns become.
Another reshaping process devised particularly for draw and ironed cans has been disclosed in Japanese Patent Laid-Open Application No. 75559/1985. According to this process, a drawn and ironed can body manufactured from an aluminum alloy sheet is subjected to a heat treatment for improvement of its mechanical elongation and expansion capability. However, this process has a disadvantage in that it essentially results in substantial increase of costs of manufacturing the drawn and ironed cans due to a heat treatment process to be added.
For the reasons thus far discussed, reshaping processes have not been used commercially for drawn and ironed cans and have found only a limited use in three piece cans of specific designs, such as a barrel shaped can, which do not need to stress highly conspicuous hill and valley configurations.